The self-reacting friction stir welding (SR-FSW) process is a primary method used by NASA to construct the Space Launch System (SLS) vehicle. This method uses large scale shear to plastically deform and mechanically mix base materials. This solid-state process causes the fabricated material to reach a temperature below the melting point, and as such, there are lower residual stresses and less warping than that observed in traditional fusion welding processes. The process parameters responsible for heat generation in the SR-FSW process include: the tool rotational speed, the tool translational speed, the crown plunge force, and the root/pin reaction force. Optimization of these process parameters is required to produce sound welded joints with the appropriate microstructural constituents. This work characterizes the effect of SR-FSW on AA2219-T87. Specifically, the material’s microhardness, strength, and θ-phase evolution are studied as a function of time and temperature. These data sets are compared to the microhardness in the friction stir weld stir zone, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base material. Additionally, residual stresses produced as a function of the friction stir welding process are quantified. Furthermore, fatigue crack growth rate data and plane-strain fracture toughness data of the welded material are compared to that of the base material. Finally, all data collected is used to calibrate a tool that determines the relative contribution from various strengthening mechanisms to the overall strength of the weld.